1. Field of the Invention
This invention relates to a steering system for a boat propelling apparatus such as an outboard motor or an inboard/outboard motor.
2. Description of the Background Art
Among the prior art steering systems for boat propelling apparatus are those disclosed in Tokkai Hei 2-147497 (U.S. Pat. No. 4,976,639) and U.S. Pat. No. 4,595,370. In such a steering system, a propelling unit having a propeller is steerably supported on the boat, and a steering piston-and-cylinder assembly is secured to the boat. The steering piston-and-cylinder assembly has its piston rod coupled to a steering arm which can steer the propelling unit. An operation cable which is coupled to a steering wheel provided in the boat, is secured to the steering arm. The operation cable operates a spool valve in an oil hydraulic operating fluid switching unit for switching the flow of operating fluid supplied to the steering piston-and-cylinder assembly. The operating force of the steering wheel is transmitted to the operation cable, whereby the spool valve of the operating fluid switching unit is moved to switch the flow of operating fluid to the steering piston-and-cylinder assembly. Thus, the propelling unit is steered by the operating fluid pressure of the steering piston-and-cylinder assembly.
In the above steering system for the boat propelling apparatus, however, the operating fluid switching unit has a switching section comprising a spool valve. This means that a play for the sliding of the spool is formed between the spool and a sleeve. Therefore, when an external force is applied to the propelling unit at the time of generation of a steering assist force by the steering piston-and-cylinder assembly, the operating fluid may be caused to flow through the play, thus making the steering unstable.
In the above steering system for the boat propelling apparatus, an oil hydraulic operating fluid supply pump which supplies oil hydraulic operating fluid through the oil hydraulic operating fluid switching unit to the steering piston-and-cylinder assembly, is stopped in the absence of any operating force being applied to the steering wheel, and is started upon application of the operating force. Therefore, at the time of the start of the operating fluid supply pump, operating fluid is caused to flow suddenly into the steering piston-and-cylinder assembly, thus preventing or interfering with a smooth increase of the steering assist force generated in the steering piston-and-cylinder assembly.
Further, as for the start and stop of the above operating fluid supply pump, stringently the oil hydraulic operating fluid supply pump is not started simultaneously with the application of the operating force to the steering wheel, but some delay time is involved due to the mechanical structure. This may result in reduction of the response of the assist force generated in the steering piston-and-cylinder assembly.